<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>id 选择器</title>
    <style type="text/css">
        #bold {font-weight:bold;}
        #font24 {font-size:24px;}
    </style>
</head>
<body>
    <p id="bold">段落1 设置粗体文字。</p >
    <p id="font24">段落2 设置字号为24px。</p >
    <p id="font24">段落3 设置字号为24px。</p >
    <p id="bold font24">段落4 设置粗体文字，设置字号为24px。</p >
</body>
</html>